1. Field of the Invention
The present invention relates to a display apparatus having organic electroluminescence elements (hereinafter referred to as organic EL elements) that emit light when a current is injected.
2. Description of the Related Art
Recently, a display apparatus employing organic EL elements has been intensely researched. An organic EL display has advantages, such as a high luminance, a wide viewing angle, and a low power consumption, and has gotten much attention as an alternative to a liquid crystal display.
An organic EL display apparatus includes a plurality of organic EL elements, which need to be individually driven. In general, when an organic EL element emits light, a current is supplied from a power source disposed outside a display area where organic EL elements are disposed, and then the current flows to the ground potential. That is to say, a current flows between both electrodes of an organic EL element disposed between the supply potential and the ground potential, and thereby a light emission is obtained. There are two technologies to drive organic EL elements: so-called passive matrix technology and so-called active matrix technology. In the passive matrix technology, the intersections of crisscrossing stripe-like electrodes are sequentially driven. In the active matrix technology, each organic EL element is provided with a thin film transistor (TFT), which controls driving the element.
Japanese Patent Laid-Open No. 8-54836 discloses an active matrix light-emitting element. The light-emitting element includes an electron injection electrode connected to the drain electrode of a TFT via a contact hole, and a hole injection electrode opposed thereto. The hole injection electrode is a layer of indium tin oxide (ITO), which is a transparent conductive material. Light is extracted from the side of this electrode. An organic thin film layer disposed between the opposing electrodes is supplied with a current controlled by the TFT, thereby emitting light.
The interconnect part connecting a circuit determining the supply potential and an organic EL element and the interconnect part connecting a circuit determining the ground potential and an organic EL element have electrical resistance. Therefore, voltage drop occurs in the interconnect parts and affects the current supplied to the organic EL element. The voltage drop in the interconnect parts is noticeable particularly in a high-definition display apparatus having thin interconnects or a large-sized screen display apparatus having long interconnects.
In addition, the effect of the resistance of the interconnect parts is noticeable particularly in the case where, in an active matrix display apparatus, the light-extraction side electrode is a transparent conductive film such as ITO and a common electrode shared by pixels. When an electrode is shared by pixels, the electrode functions as an interconnect connecting to a circuit disposed outside the display area. That is to say, when a pixel is turned on, a current flows through the part of the electrode corresponding to the other pixels. When the electrode is formed of a material having a resistance larger than that of a metal film, for example, when the electrode is formed of a transparent conductive film, voltage drop is noticeable in the electrode part and affects the current supplied to the pixel part.
Similarly, voltage drop can occur also in a passive matrix display apparatus. In the case of a passive matrix display apparatus, electrodes are stripe-like in shape. When a pixel is turned on, the part of an electrode corresponding to the other pixels functions as an interconnect. Therefore, when the electrode is formed of a material having a large resistance, for example, a transparent conductive material, voltage drop is noticeable in the electrode part and affects the current supplied to the pixel part.
As described above, the current supplied to an organic EL element is affected by the electrical resistance of the interconnect parts. This effect depends on the position of a pixel in the display area. In general, the closer a pixel to the circuit disposed outside the display area, the smaller the voltage drop which occurs in the interconnect, and, therefore, the larger the current which is supplied to the pixel. The more distant the pixel from the circuit, the larger the voltage drop which occurs in the interconnect, and, therefore, the smaller the current which is supplied to the pixel. Therefore, the central part of the display area is darker and the peripheral part of the display area is brighter. The uneven display which occurs thus deteriorates the display quality.